Early detection of specific classes of pathogens is accomplished by the innate immune system with the help of pattern recognition receptors (PRRs). The detected pathogens include viruses, bacteria, protozoa and fungi, and each constitutively expresses a set of class-specific, mutation-resistant molecules called pathogen-associated molecular patterns (PAMPs). These molecular markers may be composed of proteins, carbohydrates, lipids, nucleic acids or combinations thereof, and may be located internally or externally. Examples of PAMPs include bacterial carbohydrates (lipopolysaccharide or LPS, mannose), nucleic acids (bacterial or viral DNA or RNA), peptidoglycans and lipotechoic acids (from Gram positive bacteria), N-formylmethionine, lipoproteins and fungal glucans.
Pattern recognition receptors have evolved to take advantage of three PAMP qualities. First, constitutive expression allows the host to detect the pathogen regardless of its life cycle stage. Second, the PAMPs are class specific, which allows the host to distinguish between pathogens and thereby tailor its response. Third, mutation resistance allows the host to recognize the pathogen regardless of its particular strain.
Pattern recognition receptors are involved in more than just recognition of pathogens via their PAMPs. Once bound, pattern recognition receptors tend to cluster, recruit other extracellular and intracellular proteins to the complex, and initiate signaling cascades that ultimately impact transcription. Additionally, pattern recognition receptors are involved in activation of complement, coagulation, phagocytosis, inflammation, and apoptosis functions in response to pathogen detection.
Pattern recognition receptors (PRRs) may be divided into endocytic PRRs or signaling PRRs. The signaling PRRs include the large families of membrane-bound Toll-like receptors (TLRs) and cytoplasmic NOD-like receptors, while the endocytic PRRs promote the attachment, engulfment and destruction of microorganisms by phagocytes without relaying an intracellular signal, are found on all phagocytes and mediate removal of apoptotic cells. In addition, endocytic PRRs recognize carbohydrates and include mannose receptors of macrophages, glucan receptors present on all phagocytes and scavenger receptors that recognize charged ligands.